The invention relates generally to the field of polyphase motor control systems, and more specifically to the field of such systems in which DC operating potential provides operative power for the polyphase motor.
In prior polyphase, meaning two or more phase, motor control systems which receive DC operative power for a motor from a DC power supply, handling the power transient signals produced by the motor phase windings is generally a problem. Some prior systems, such as the full wave or full bridge motor control systems, utilize pairs of upper and lower switching devices and pairs of upper and lower transient (flyback) diodes to allow the return of the inductive energy stored in the windings to either other windings or the operative DC supply. In such systems, each pair of upper and lower drive devices is directly connected across the DC power supply potential. Such prior systems are able to adequately function when the switching devices receive pulse width modulated pulse excitation which is used to control and/or determine motor rotation speed. However, such control systems result in providing substantial ripple current contamination of the DC power supply and substantial filtering is required to attempt to minimize pulsations in the DC operating potential caused by device switching and commutation transients. Such pulsations in the battery voltage are undesirable if additional loads, besides the polyphase motor, are to be connected across the same voltage, such as would be the case for a polyphase motor receiving operating potential from a battery in an automobile.
Some prior systems have attempted to reduce the number of controllable switching devices used in polyphase motor controls. When this is done, some of these prior systems have developed a boost or storage voltage which may or may not be returned to the DC operating potential depending upon the configuration of these prior systems. The systems which do not return the stored energy to the operating potential or motor windings, are power inefficient and therefore undesirable. Systems which attempt to return the stored inductive energy to the power supply and/or motor, require the use of an additional nondissipative energy storage element, such as an inductor, and typically a chopper control circuit. These components increase the cost of such systems. Also, the use of a chopper control circuit results in generating additional signal transients and that results in undesired radiated noise. In addition, such systems are not operative over a large range of duty cycles for pulse width modulated control signals applied to the switching devices and are difficult to control so as to substantially reduce battery voltage ripple.
In prior polyphase motor control systems, typically all of them result in the phase commutation transients substantially affecting battery current, and therefore battery voltage. In order to minimize the effect of these very lower frequency transients, extensive filtering by large value capacitors and/or inductors is generally utilized, and of course this is undesirable.